Multiturn rotary encoders are used in drive technology to measure the position as well as the number of revolutions performed by a shaft. Such multiturn rotary encoders have been known for a long time in the prior art. They are usually made up of a first measuring standard, by scanning which, information is obtained about the position of the shaft within one complete revolution, and one or more further measuring standards, whose scanning supplies position information about the number of revolutions performed by the shaft.
The units for determining position within one revolution of the shaft are combined under the term “singleturn stage.” The units for ascertaining position information regarding the number of revolutions performed by the shaft form a multiturn unit. From the position information of the singleturn stage and the multiturn unit, an evaluation unit forms a combined position value which includes both the number of complete revolutions performed up to now, as well as the absolute position within the instantaneous revolution.
In modern position-measuring devices, a large-scale-integrated, application-specific component (ASIC) is often used as evaluation unit, which in the ideal case, already includes the detectors for scanning the singleturn stage, as well as the circuit elements necessary for processing the detector signals to form the position value. If an optical scanning principle is used, for example, the detectors are in the form of a photodetector array. In this combination, one also speaks of an opto-ASIC.
To scan the second measuring standards in the multiturn unit, separate scanning units are provided which generate position values in absolute form. For example, the multiturn unit of a typical multiturn rotary encoder has three multiturn stages, which are driven via a three-stage gear unit having a fixed gear-ratio factor. Magnets may be used as measuring standards, which are mounted axially on gear wheels and which in each case are scanned by a Hall sensor that outputs the angular position of the assigned gear wheel as an absolute value with, e.g., 8-bit resolution. These absolute values are transmitted via digital interfaces to the evaluation unit. A multiturn rotary encoder of this type is described, for example, in European Patent No. 1 076 809.
Multiturn rotary encoders are also known whose multiturn unit has a counter that is incremented or decremented by counting pulses, which are generated upon each complete revolution of the shaft. The count value in this case directly represents the number of revolutions performed by the shaft. A typical data word length of the count value is 18 bits.
In known methods heretofore, preferably parallel interfaces were used for transmitting data from the multiturn unit to the evaluation unit, since they are easy to implement and achieve high data-transmission rates. The disadvantage in this design approach is the high number of signal lines required. Thus, the three-stage system described above alone requires 24 lines for transmitting the three 8-bit long data words. Added to this are also various control lines. This is particularly problematic when an ASIC or opto-ASIC is to be used as evaluation unit, since the number of terminal pads needed directly influences the chip area and therefore the costs per ASIC.
The use of serial interfaces likewise proves to be problematic, since simple serial interfaces, which can be implemented with low expenditure in an ASIC, are usually too slow for exacting position-measuring devices, e.g., for measuring high-speed spindles, and fast serial interfaces require great circuit complexity and therefore again have a negative effect on the chip area needed and lead to increased costs. Moreover, fast serial interfaces require a high clock-pulse rate with steep clock-pulse edges that may give rise to problems due to crosstalk to the signal processing, especially of the singleturn stage, and lead to high current consumption because of the required drive strength of the pads.